The invention relates to a system and method for controlling an internal combustion engine coupled to an emission control device.
In direct injection spark ignition engines, the engine operates at or near wide-open throttle during stratified air-fuel ratio operation in which the combustion chambers contain stratified layers of different air-fuel ratio mixtures. Strata closest to the spark plug contain a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during its intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to emission control devices known as three-way catalytic converters optimized to reduce CO, HC, and NOx. When operating at air-fuel ratio mixtures lean of stoichiometry, a three way catalyst optimized for NOx storate, known as a NOx trap or catalyst, is typically coupled downstream of the first three-way catalytic converter. The NOx trap typically stores NOx when the engine operates lean and release NOx to be reduced when the engine operates rich or near stoichiometry.
One method of determining degradation of the NOx trap uses an estimate of NOx trap capacity. In this approach, NOx trap capacity is found by performing lean operation for a long enough duration so that the trap is completely filled with NOx. Then, the stored NOx and stored oxygen are purged. From total purge fuel, a total of oxygen storage and NOx storage is found. Then, using total storage and a previously calculated oxygen storage estimate, NOx capacity is found as the difference between the two. The oxygen storage estimate is found by performing lean operation for a short enough duration so that almost no NOx is stored in the trap. Then, by purging the stored oxygen, oxygen storage can be found from the purge fuel. Such a method is described in U.S Pat. No. 5,713,199.
The inventors herein have recognized a disadvantage with the above approach. In particular, estimates of NOx capacity found this way are sensitive to errors in purge fuel used. For example, oxygen storage capacity can be over half of the total storage capacity of a NOx trap. Thus, since a relatively small difference between two purge fuel calculations are needed to find NOx storage, small errors in purge fuel used may results in significant errors in estimated NOx capacity. In other words, known prior methods have a low signal to noise ratio.
Another approach to determine degradation of the NOx trap uses a difference between NOx concentration entering the trap and NOx concentration exiting the trap. When this difference less than a predetermined value, the trap is judged to be degraded. Alternatively, a ratio of NOx concentration entering the trap and NOx concentration exiting the trap could also be used. Such a method is described in U.S. Pat. No. 5,953,907.
The inventors herein have recognized a disadvantage with the above approach. In particular, the difference between NOx concentration entering the trap and NOx concentration exiting the trap depends on the amount of NOx stored in the trap. For example, when the trap is nearly filled with NOx, little additional NOx can be stored in the trap. Thus, even though the trap performance is not degraded, an indication will be generated since there is a small difference between NOx concentration entering the trap and NOx concentration exiting the trap.
An object of the invention claimed herein is to provide a method for determining capacity of a NOx trap coupled to an internal combustion engine. The above object is achieved, and disadvantages of prior approaches overcome, by a method for determining storage of an emission control device coupled to an internal combustion engine with a nitrogen oxide sensor coupled to an exhaust gas of the engine downstream of the emission control device, the engine coupled to a vehicle, the method comprising: determining nitrogen oxides entering the emission control device based on engine operating conditions; determining nitrogen oxides exiting the emission control device based the sensor; and calculating nitrogen oxides stored in the emission control device based on an accumulated difference between said nitrogen oxides entering the emission control device and nitrogen oxides exiting the emission control device.
By using a combination of estimated NOx entering the emission control device and measured NOx exiting the emission control device, it is possible to calculate capacity without knowing or estimating oxygen storage. Further, it is possible to calculate capacity without relying on purge fuel used.
An advantage of the above aspect of the present invention is improved estimates of NOx trap capacity by eliminating uncertainty in purge fuel used.
Another advantage of the above aspect of the present invention is less complex estimation since it is not necessary to estimate oxygen storage.
Yet another advantage of the above aspect of the present invention is improved emissions since a complete NOx trap fill is not required.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.